Some conventional radio frequency (RF) transceivers include a full duplex functionality, allowing RF transmission simultaneously with RF reception, although at different frequencies. Focusing on the receive side, one example conventional RF receiver receives an RF signal over the air and passes the RF signal to a low noise amplifier (LNA). The output of the LNA may be passed to a transformer. In other words, the RF signal may be received as a single-ended signal, and the transformer creates a differential signal to pass to a mixer. The mixer then down converts the RF signal to a baseband signal. The receive side then performs further amplification, filtering, and digital conversion. A potential drawback of such a circuit is that the transformer may be relatively narrow-band because it may include capacitive and inductive components tuned to a particular target frequency. Operation outside of the relatively narrow band around the target frequency and/or implementing tuning over a wide bandwidth may cause unacceptable loss of performance. Another potential drawback of such a circuit is that its poor reverse isolation prevents a single LNA from directly driving two or more transformers and downconverter paths in carrier aggregation architectures. Isolation between downconverters in carrier aggregation receive architectures is necessary such that intermodulation products from jammers and two or more local oscillators do not corrupt the received signal. Furthermore, it may be difficult and expensive to implement coupled inductors of the transformer on a semiconductor chip.
Another conventional RF receiver uses active baluns to create the differential signals. Generally, active baluns do not have the narrow-band limitations of conventional transformers because they may be implemented using transistors and may omit or significantly reduce the use of capacitive or inductive components. However, active baluns are nonlinear devices that may experience intermodulation distortion (IMD). Specifically, active baluns experience second-order and third-order IMD, where some of those IMD components may fall within the receive signal band and make it difficult to recover the received signal. Specifically, in a full-duplex transceiver, some of the energy of the transmitted signal may inevitably find its way into the receiver. Third-order IMD of that transmitted signal and undesired signals adjacent to the receive channel may include frequencies that fall within the receive signal band. In other words, third order IMD may cause distortion that, unless reduced or prevented, degrades the received signal.
Accordingly, there is a need in the art for an RF receive circuit with a relatively wide band of operation and relatively low susceptibility to IMD.